There is a long-standing need for valve devices which can be used to produce a variable flow. Varying the flow through valves is usually achieved by varying the available cross-sectional area presented to the flow. Devices utilizing magnetic elements attached to a valve stem, which co-operate with electromagnetic coils in order to accurately align a valve element and thus control the rate of flow through a valve are well-known in the art. A typical example of this type of valve is disclosed in U.S. Pat. No. 5,509,439, where the power to a single electromagnetic coil is varied, causing a magnetic element to push against a spring. The force the element exerts increases or decreases with changes in the power to the coil and this changes the cross-sectional area of the flow path.
An example of a similar device is disclosed in U.S. Pat. No. 2,910,089. The power to a coil is altered in order to alter the position of a magnetic element, with a secondary coil used to hold or lock the valve in the desired position.
A difficulty with these devices resides in the difficulty of accurately controlling the position of the magnetic element, and achieving a graduated flow variation profile, particularly at the low flow rate end of the control.